The present invention relates to a gas purification unit. In particular, the present invention relates to a gas purification unit for purifying a gas before said gas is dispensed from a liquefied gas container such as a cylinder.
Many commercial processes, particularly in the electronics and semi-conductor industries, require high purity gases and gas mixtures, i.e. gases and gas mixtures that are substantially free from one or more unwanted components (xe2x80x9cimpuritiesxe2x80x9d). In many applications, the high purity gases and gas mixtures must contain less than a total of 50 parts per million (xe2x80x9cppmxe2x80x9d) nitrogen, oxygen, carbon monoxide, carbon dioxide, hydrocarbon compounds and water. In other applications, the only requirement may be that the high purity gas or gas mixture contains less than a few ppm of a specific impurity, for example water. In certain cases, an unacceptable level of impurities may be of the order of several parts per billion (109) (xe2x80x9cppbxe2x80x9d).
Moisture is recognized by the semiconductor industry as a key impurity, the presence of which has a direct impact on the yield of semiconductor devices. It is also generally acknowledged as the most difficult impurity to remove effectively from gas distribution systems. The industry trend is moving towards larger semiconductor wafer diameters which in turn will lead to lower process pressures and gas flows. Under these conditions, the moisture adsorbed on to the inner surfaces of the processing equipment is readily desorbed into the gas stream affecting the performance of the semiconductor manufacturing process.
Corrosive gases used in semiconductor manufacturing processes, particularly hydrogen chloride (xe2x80x9cHClxe2x80x9d), are often sources of moisture contamination. HCl supplied in containers for use in semiconductor manufacturing applications has a moisture specification of 1 ppm (VLSI grade) or 2 ppm (electronic grade). It is at high pressure in liquid form as supplied. As it is consumed, the liquid HCl boils off to form the gaseous HCl which is supplied to the process. After most of the liquid has evaporated, the last few percent of liquid and gas remaining is known as the xe2x80x98heelxe2x80x99. While the bulk of the contents of the container will meet the moisture specification, there is a significant increase in the moisture concentration as the container heel is used and this moisture specification may well be exceeded which could have a significant and adverse effect on the semiconductor manufacturing process. FIG. 6 depicts how the measured moisture concentration increases as the container heel is used.
One way of tackling moisture contamination from a gas is to remove moisture from the gas at the point of use (xe2x80x9cPOUxe2x80x9d) of the gas.
U.S. Pat. No. 5,403,387 (Flynn et al; published on Apr. 4, 1995) discloses a device for removing moisture from a flow of gas to provide air or other gases having relatively low humidity. The device comprises an elongated housing and a connection head which together form a canister. The connection head defines a gas inlet, a gas outlet and a closure surface. The gas inlet is connected to a distribution tube that extends towards the bottom of the interior of the canister. A desiccant material, preferably one which can be rejuvenated by baking, e.g. silica gel, is provided within the housing around the distribution tube. Gas travels into the housing via the gas inlet and the distribution tube, passes through the desiccant material where it is dried and then leaves the device via the gas outlet. The device is designed to be used outside a container of the gas to be dried. In the only exemplified application, the device is used to dry a compressed air supply for a spray painting process.
EP-A-0288931 (Glenn; published on Nov. 2, 1988) discloses a valve block that is joinable to a receptacle to form a container suitable for use in the semiconductor industry primarily for liquid storage/vapour dispensing but also for gas purification applications. The valve block is characterised by having a triple valve configuration that permits dead space gases to be readily purged from a gas distribution system by an integral purge by-pass loop. In gas purification applications, the container is filled with a sorbent material which, in the only example provided, is an arsine scavenger. A dip tube extends downwardly from the valve block to direct the gas to be purified to the bottom of the sorbent material bed. The gas travels through the sorbent material and exits the container via a valve in the valve block. For all applications, the container is located outside the gas container containing the gas to be purified.
U.S. Pat. No. 5,409,526 (Zheng et al; published on Apr. 25, 1995) discloses apparatus for supplying high purity gas comprising a container having a valve with two internal ports. One internal port is used to fill the container while the other is fitted with a gas purification unit that extends into the container and which removes particulates and impurities from the gas as it leaves the container.
An example of the type of gas purification unit disclosed in Zheng is shown in FIG. 1. The unit 102 comprises a tubular body 104 having a first end 106 and a second end 108. The first end 106 has a gas outlet 110 that is adapted for connection to a valve and the second end 108 has a gas inlet 112. The unit 102 further comprises a filter disc 114 adjacent the inlet that is held in place with a circlip 116. Purification of the gas is provided by the combination of a layer 118 of an adsorbent material sandwiched between two layers 120, 122 of molecular sieves. This combination of layers is held between two plugs 124, 126 of glass wool. The gas outlet 110 is provided with a stainless steel filter 128.
In use, gas enters the unit 102 through the mouth of the gas inlet 112 at the bottom of the unit. The gas passes through the filter disc 114, the first glass wool plug 124, the various layers 118, 120, 122 of purifying material, the second glass wool plug 126 and the steel filter 128 and exits through the gas outlet 110 at the top of the unit whereupon it leaves the container via the valve assembly.
In EP-A-0916891 (Zheng et al; published on May 19, 1999), there is disclosed a gas control device for use with a container of compressed gas. The gas control device may comprise a purifier unit that is located within the container. It is disclosed that the purifier unit may conveniently be as described in U.S. Pat. No. 5,409,526.
Although this arrangement works well with permanent gases, there are several disadvantages associated with the gas purification unit disclosed in Zheng when working with liquefied gases. One of these disadvantages is that if the level of liquefied gas within a container is above the mouth of the gas inlet, the unit would act as a dip tube and allow liquefied gas to escape from the container.
FIG. 5a depicts a gas purification unit 502a of the type shown in FIG. 1 in situ attached to a valve assembly 506a mounted on a liquefied gas container 504a. The valve assembly includes internal pressure regulation. The inlet 512a is located at the lower end of the unit 502a and, thus, if the lower end of the unit is submerged in the liquefied gas, then liquefied gas is drawn from the container 504a. Liquefied gas withdrawn in this way could cause unwanted and possibly dangerous flooding of the gas distribution system. Such flooding would effect productivity not only by possibly ruining a batch of articles under production but also by requiring costly and time consuming clean up. The downtime of the process apparatus during clean up could have a significant effect on productivity.
These problems may be overcome by ensuring that the level of liquefied gas in the container is not higher than the mouth of the gas inlet of the purification unit. This can be achieved by partially filling the container thereby requiring use of a larger container if the same amount of liquefied gas is to be stored.
However, solutions of this type may not be cost effective. Partially filling a container means that the container needs to be replaced sooner (thereby increasing transportation costs and process apparatus downtime, etc.) and using a larger container would be wasteful and inefficient. As a conventional gas purification unit can extend a significant fraction of the length of the liquefied gas container, a significant of the container will be empty.
It is the primary object of the present invention to provide a gas purification unit that can be used with conventional liquefied gas containers that overcomes the above-mentioned problems without incurring any significant cost penalties.
In a first aspect of the present invention, there is provided a gas purification unit suitable for purifying a gas before said gas is dispensed from a liquefied gas container having a valve assembly for discharging gas from the container, said unit comprising:
a hollow body having a first end and a second end and being at least partially filled between said ends with gas purifying material;
a gas outlet provided at the first end of the body and adapted for connection to said valve assembly so that the body extends into the container; and
gas inlet means permitting gas flow from the container to the second end of the body,
wherein the second end of the body is closed and the gas inlet means comprises conduit means having a mouth at or near the first end of the body.